Abstract

Radio-frequency (rf) superconducting cavities made of high purity niobium are widely used to accelerate charged particle beams in particle accelerators. The major limitation to achieve rf field values approaching the theoretical limit for niobium is represented by “anomalous” losses which degrade the quality factor of the cavities starting at peak surface magnetic fields of about , in the absence of field emission. These high field losses are often referred to as drop. It has been observed that the drop is drastically reduced by baking the cavities at for about under ultrahigh vacuum. An improved oxygen diffusionmodel for the niobium-oxide system is proposed to explain the benefit of the low-temperature baking on the drop in niobiumsuperconducting rf cavities. The model shows that baking at for allows oxygen to diffuse away from the surface, and therefore increasing the lower critical field towards the value for pure niobium.

Received 18 August 2005Accepted 01 June 2006Published online 13 July 2006

Acknowledgments:

The authors would like to acknowledge C. Antoine, J. Delayen, J. Halbritter, P. Kneisel, and Q. Ma for many useful discussions. This work was supported by the U.S. DOE Contract No. DE-AC05-84ER40150, Modification No. M175, under which the Southeastern Universities Research Association (SURA) operates the Thomas Jefferson National Accelerator Facility.